In a CCD type solid-state imaging device or a CMOS type solid-state imaging device mounted in a digital camera, a large number of photoelectric conversion devices (photodiodes) serving as photo acceptance portions and signal readout circuits for reading out photoelectric conversion signals obtained by the photoelectric conversion devices to the outside are formed on a surface of a semiconductor substrate. In the CCD type solid-state imaging device, each of the signal readout circuits includes a charge transfer circuit, and a transfer electrode. In the CMOS type solid-state imaging device, each of the signal readout circuits includes an MOS circuit, and a signal wiring.
Accordingly, in the solid-state imaging device according to the related art, both the large number of photo acceptance portions and the signal readout circuits have to be formed together on the surface of the semiconductor substrate. There is a problem that the total area of the photo acceptance portions cannot be enlarged.
In addition, in a single plate type solid-state imaging device according to the related art, one of color filters, for example, of red (R), green (G) and blue (B) is stacked on each photo acceptance portion so that each photo acceptance portion can detect an optical signal with corresponding one of the colors. For this reason, for example, a blue optical signal and a green optical signal in a position of a photo acceptance portion for detecting red light are obtained by applying an interpolation operation on detection signals of surrounding photo acceptance portions for detecting blue light and green light. This causes false colors to thereby result in lowering of resolution. In addition, blue and green light beams incident on a photo acceptance portion covered with a red color filter are absorbed as heat to the color filter without giving any contribution to photoelectric conversion. For this reason, there is also another problem that light utilization efficiency deteriorates and sensitivity is lowered.
While the solid-state imaging device according to the related art has various problems as described above, development on increase in the number of pixels has advanced. At present, a large number of photo acceptance portions (e.g. equivalent to several million pixels) are integrated on one chip of a semiconductor substrate, so that the size of an aperture of each photo acceptance portion approaches the wavelength of light. Accordingly, it is difficult to expect a CCD type or CMOS type image sensor to have better image quality or sensitivity than ever to thereby solve the abovementioned problems.
Under such circumstances, the structure of a solid-state imaging device, for example, described in JP-A-58-103165 has been reviewed. The solid-state imaging device has a structure in which a photosensitive layer for detecting red light, a photosensitive layer for detecting green light and a photosensitive layer for detecting blue light are stacked on a semiconductor substrate having signal readout circuits formed in its surface, by a film-forming technique and in which these photosensitive layers are provided as photo acceptance portions so that photoelectric conversion signals obtained by the photosensitive layers can be taken out to the outside by the signal readout circuits. That is, the solid-state imaging device has a photoelectric conversion film-stacked type structure.
According to the structure, limitation on design of the signal readout circuits can be reduced greatly because it is unnecessary to provide any photo acceptance portion on the surface of the semiconductor substrate. Moreover, sensitivity can be improved because efficiency in utilization of incident light is improved. In addition, resolution can be improved because light with the three primary colors of red, green and blue can be detected from one pixel. The problem of false colors can be eliminated. The problems inherent to the CCD type or CMOS type solid-state imaging device according to the related art can be solved.
Therefore, photoelectric conversion film-stacked type solid-state imaging devices described in JP-A-2002-83946, JP-T-2002-502120, JP-T-2003-502841 and JP-B-3405099 have been proposed in recent years. An organic semiconductor or nano particles may be used as the material of each photosensitive layer.
However, when one of such photoelectric conversion film-stacked type solid-state imaging devices is used, the structure of the semiconductor substrate where the signal readout circuit is formed is not good enough for capturing a high-resolution still image, so that there is a problem that noise is often superposed on the readout signal. This problem will be described with reference to FIGS. 13A to 13C.
FIG. 13A is a typical sectional view of main part in a surface portion of a semiconductor substrate, in which a charge-coupled element type of charge transfer channel is used as a signal readout circuit. A signal charge-storage portion 2 made of an n-type semiconductor region is provided in a surface portion of a semiconductor substrate 1. A charge transfer channel 3 made of an n-type semiconductor region is provided in the surface portion of the semiconductor substrate 1 so as to be separate from the signal charge-storage portion 2. The outmost surface of the semiconductor substrate 1 is covered with an electrically insulating film 4. A columnar wiring electrode 5 piercing the electrically insulating film 4 is provided so as to be erected from the signal charge-storage portion 2 and connected to a corresponding electrode (not shown) of a photoelectric conversion film. A transfer electrode (readout electrode) 6 bridging between the signal charge-storage region 2 and the charge transfer channel 3 is provided on the surface of the electrically insulating film 4.
FIG. 13B is a view showing a potential well in the configuration depicted in FIG. 13A. Since the columnar wiring electrode 5 is integrally connected to the signal charge-storage portion 2, a large number of free electrons QB are present in a well 7 formed under the signal charge-storage portion 2. Signal charges Qsig generated by photoelectric conversion flow through the wiring electrode 5 onto the free electrons QB.
When a readout pulse is applied to the transfer electrode 6, a barrier 9 between a well 8 under the charge transfer channel 3 and the well 7 is moved down as shown in FIG. 13C so that the signal charges Qsig are transferred from the well 7 to the well 8. Then, the signal charges Qsig are read out. As a result, only the free electrons QB remain in the well 7.
If the free electrons QB in the well 7 are always constant, that is, if the potential of the potential barrier 9 at the time of application of the readout pulse is always constant, no problem will be caused. When the potential V1 of the readout pulse applied to the transfer electrode 6 fluctuates by ΔV1 during reading out of the signal charges Qsig, the free electrons QB may be partially transferred into the well 8 so that there is a problem that the transferred free electrons QB are superposed as noise on signal charges.
As described above, the photoelectric conversion film-stacked type solid-state imaging device has such a problem that a large number of free electrons present in a wiring electrode portion for connecting an electrode of a photoelectric conversion film and a signal charge-storage portion to each other are read out as noise together with signal charges into a charge transfer channel because of potential fluctuation of a readout pulse. If the photoelectric conversion film-stacked type solid-state imaging device is mounted in a digital camera etc. while the problem remains unsolved, it is impossible to capture a high-resolution and high-quality still image.
Since an overflow drain structure for achieving an electronic shutter is not provided in the photoelectric conversion film-stacked type solid-state imaging device according to the related art, it is necessary to make a research on how to include the structure in the photoelectric conversion film-stacked type solid-state imaging device according to the related art.